Volume 13 Number 14 14 April 2015 Pages 4117–4354
Organic &
Biomolecular
Chemistry
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ISSN 1477-0520
COMMUNICATION
Sofia I. Pascu, Yun-Bao Jiang, Tony D. James et al.
Synthesis and evaluation of a boronate-tagged 1,8-naphthalimide probe
for fluoride recognition
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Biomolecular Chemistry
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Cite this: Org. Biomol. Chem., 2015,
13, 4143
Received 27th October 2014,
Accepted 4th December 2014
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Synthesis and evaluation of a boronate-tagged
1,8-naphthalimide probe for fluoride recognition†
Su-Ying Xu,a Xiaolong Sun,a Haobo Ge,a Rory L. Arrowsmith,a John S. Fossey,b
Sofia I. Pascu,*a Yun-Bao Jiang*c and Tony D. James*a
DOI: 10.1039/c4ob02267j
www.rsc.org/obc
A biocompatible fluoride receptor has been developed where the
interaction between the boronic acid ester and amine (NH) results
in fluoride ion selectivity and enhanced fluorescence quenching.
Introduction
Anion recognition is one of the most challenging problems for
analytical chemists due to the complexity of their geometries,
small charge to radius ratios and heavy solvation.1–3 Synthetic
molecular strategies for anion detection require a flexible
design and matching of binding site geometries to specific
anions. Several reviews summarise the development of anion
chemosensors.2,4–6 Fluoride recognition has attracted substantial interest not only because of its unique properties but also
its importance in our daily life. In particular, fluoride salts can
be used as phosphatases inhibitors, because they mimic the
structure of the phosphate group and therefore act as transition state analogues.7 Also, while low levels of fluoride anion
can be used for oral hygiene,8,9 an excess of fluoride results in
fluorosis.10 Many fluoride chemosensors have been developed
based on the hydrogen bonding interaction between fluoride
and the –NH groups of (thio)ureas,11–15 amides,16,17 and
pyrrole moieties.18,19 While, the strong Lewis base character of
fluoride can be used in sensing through coordination with
Lewis acidic boron atoms.20–27 Recently, several chemodosimeters for fluoride anions have been developed exploiting the
strong affinity of silicon with fluoride.28–30 While many receptors have been used independently, very few examples exist
a
Department of Chemistry, University of Bath, Bath BA2 7AY, UK.
E-mail: t.d.james@bath.ac.uk, s.pascu@bath.ac.uk
b
School of Chemistry, University of Birmingham, Edgbaston, Birmingham,
West Midlands B15 2TT, UK
c
Department of Chemistry, College of Chemistry and Chemical Engineering, and the
MOE Key Laboratory of Analytical Sciences, Xiamen University, Xiamen 361005,
China. E-mail: ybjiang@xmu.edu.cn
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ob02267j
This journal is © The Royal Society of Chemistry 2015
where receptors motifs have been used cooperatively. Gabbai
et al. has incorporated an amide moiety with triarylboranes.
Where, the hydrogen-bond donor groups assist fluoride
binding at the boron centre, leading to a significant increase
in the stability constant of the corresponding fluoride
complex.31 Yoon and co-workers has combined an imidazole
group with a boronic ester to create a (C–H)+⋯F− ionic hydrogen-bond and boron-fluoride regime. They found that only a
receptor with an ortho-boron and imidazolium exhibited
enhanced fluoride binding.32
4-Amino-1,8-naphthalimide derivatives are often used in
the design of synthetic molecular probes, due to their versatility and easy functionalization especially in the –NH position.
There have been a number of boronic acid or ester – modified
naphthalimide derivatives reported,33–38 however, to the best
of our knowledge they have mostly focused on the detection of
monosaccharides. Herein, we prepared a boronate-tagged 1,8naphthalimide derivative 1 and reference probe 2 containing a
phenolic OH (Fig. 1) for cooperative fluoride recognition. We
also explored the potentials of using probe 1 for cellular
imaging applications.
Fig. 1
Schematic representations of probes 1 and 2.
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Results and discussion
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Synthesis and optical behaviour
Boronic ester 1 and phenol 2 were prepared following published methods.39 The anions fluoride and acetate were
titrated with probes 1 and 2 (Fig. 2 and S1–S3†). Probe 1 has
an intense absorption peak at 448 nm in dry acetonitrile
(MeCN), and displays solvent dependant properties, showing a
bathochromic shift along with decreasing solvent polarity
(Fig. S4†). Upon addition of F−, two new absorption bands
appeared at 386 nm and 590 nm, respectively, along with a
decrease of absorbance at 448 nm (Fig. 2). Probe 2 has an
intense absorption at 442 nm and addition of fluoride
induced a new absorption peak at 575 nm and a decrease of
fluorescent emission at 520 nm. The observed changes indicate that anion bindings with both 1 and 2 result in strong
hydrogen bonding with acetate and eventual deprotonation by
fluoride of the 4-amino moiety.40–42
The binding mode for fluoride and probe 1 is complicated
due to the presence of multiple binding sites. Job plot analysis
indicates a 5 : 1 binding mode (Fig. S5†), consistent with three
fluorides binding with the boron and two with the amine
hydrogen. The most relevant binding constants were evaluated
using a modified Hill equation.43 For fluoride, the binding
constant K has a value as (1.6 ± 0.03) × 104 M−1 with R2 > 0.995
(Fig. S6†). The Hill coefficient n has a value of 2, indicating
cooperative binding. For acetate Job plot analysis indicates a
1 : 1 binding mode (Fig. S7†), while the binding constant
based on Hill analysis is 7158 ± 424 M−1 with R2 > 0.998. The
Hill coefficient n value is 1, suggesting an independent
binding event (Fig. S8†).
Probe 2 is more sensitive to fluoride. 3 eq. of fluoride produces a plateau in the absorption spectra, while probe 1
requires 13 eq. of fluoride to reach a plateau (Fig. S9†). Job
plot analysis indicates a 4 : 1 binding mode (Fig. S10†), while
the binding constant based on Hill analysis is (4.3 ± 0.19) ×
104 M−1 with R2 as 0.992 (Fig. S11†). The Hill coefficient n has
Fig. 2 (a) Absorption spectra changes of probe 1 along with addition of
fluoride anion in MeCN; (b) Plot of absorption at 590 nm versus concentration of TBAF [1] = 10 μM.
4144 | Org. Biomol. Chem., 2015, 13, 4143–4148
Scheme 1 Proposed binding modes for probes 1 and 2 with fluoride
and acetate anion in MeCN. Stoichiometry of binding and Job plot
analysis: observed value and (theoretical) value.
a value of 2 indicating cooperative binding. Upon addition of
acetate to probe 2, an absorption peak at 575 nm was
observed, similar to that seen with fluoride. Probe 2 displayed
similar sensitivity toward acetate (Fig. S12†). Job plot analysis
(Fig. S13†) indicates a 2 : 1 binding mode. Binding constants
were obtained using the Hill equation giving a binding
constant K of (7.7 ± 0.25) × 104 M−1 (R2 = 0.994) (Fig. S14†)
The Hill coefficient n has a value of 2, indicating cooperative
binding.
From these observations we can propose a binding mechanism for fluoride and acetate with probes 1 and 2 (Scheme 1).
As expected the reference system probe 2 was not selective for
fluoride over acetate.
Amongst the other anions investigated with probe 1 other
than F− and OAc−, only H2PO4− produced a moderate response
(Fig. 3 and Fig. S15†). Interestingly, F− produces different spectral changes to the other anions. The original peak at 448 nm
Fig. 3 Comparison of absorption spectra after addition of 5 equivalents
of different anions in MeCN. [1] = 12 μM.
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Organic & Biomolecular Chemistry
Fig. 4 Colour changes of probe 1 after treatment with different anions.
Samples show, from left to right: 12 uM probe 1, 13 equiv. of each
anions or the anions F−, OAc−, H2PO4− respectively.
shifts to 470 nm and a new peak at 590 nm increases when
5 equiv. of F− were added and none of the other anions induce
such a red-shift. This shift from 448 nm to 470 nm is ascribed
to coordination between fluoride and the boron atom. Interestingly the different anions produce different colour changes, a
bright yellow solution of probe 1 changes colour to mauve on
addition of F−, blue grey with OAc− and tan with H2PO4−
anions as shown in Fig. 4.
Next, fluorescence titration experiments for probe 1 were
carried out with fluoride and acetate (Fig. 5 and S16†), both of
which indicate a decreased fluorescence upon addition of fluoride. Again, a red-shift of the maximum emission of probe 1
at higher concentrations of fluoride anion was observed.
A significantly better selectivity between fluoride and
acetate was observed in the fluorescence experiments (Fig. 6
(b)). This may be a synergistic effect between the boronate
anion and amine anion i.e. these two anions are better at
quenching the fluorescence of the naphthalimide than the
amine anion alone. (Scheme 1).
Communication
Fig. 6 (a) Absorption plots of probe 1 with fluoride and acetate anion
versus ratios of anion to probe 1; (b) Fluorescence emission intensities of
probe 1 with fluoride and acetate versus ratios of anion to probe 1.
Fig. 7 1H NMR titration of probe 2 with TBAF in DMSO-d6. [2] =
10.7 mM.
In order to probe further the species proposed in Scheme 1,
H NMR spectroscopic analysis was carried out in DMSO-d6,
using 20 mM of probe 1 and 10.7 mM of probe 2 at 25 °C
(Fig. 7, 8 and Tables S17 and S18†). For probe 2, as soon as
TBAF is added the initial amine NH proton (Ha) at 11.50 ppm
and the phenol OH proton (Hc) at 10.25 ppm disappear ( probably due to becoming very broad). However, for probe 1 upon
the addition of TBAF, in addition to the initial amine NH
proton’s chemical shift (Ha) at 11.66 ppm, a new peak appears
at 11.54 ppm, indicating the formation of two species. Similarly for the imine protons (Hb), the initial peak at 8.98 ppm
decreased and a new peak at 9.06 ppm appeared. After
addition of one equivalent of fluoride, the original NH peak
(Ha) reduced its intensity and was broadened. When the concentration of fluoride was further increased up to three equivalents with respect to probe 1, this signal disappeared.
Meanwhile, the original peak of the imine proton (Hb) at
8.98 ppm also disappeared leaving a peak at 9.06 ppm.
1
Fig. 5 Fluorescence spectra changes of probe 1 upon addition of TBAF
in MeCN. λex = 450 nm, [1] = 2 μM.
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Conclusions
Fig. 8
1
H NMR titration of probe 1 with TBAF in DMSO-d6. [1] = 20 mM.
A colorimetric and fluorescent fluoride probe was developed
based on a boron-modified 1,8-naphthalimide derivative. The
coordination motif between the fluoride and the boron centre,
coupled with hydrogen bonding and eventual deprotonation of
the –NH fragment, account for the enhanced selectivity toward
F− ions. Preliminary experiments to evaluate probe 1 in cellular imaging applications indicate that probe 1 may be considered a viable system for the bioimaging of prostate cancer
cells. We are currently interested in developing these types of
4-amino-1,8-naphthalimide derivatives as intracellular sensors
for a variety of substrates and explore their applications
towards multimodal optical imaging coupled with PET (Positron Emission Tomography) using rapid 18F labelling in polar
environments. While probe 1 shows some promising relevant
properties for this, its current specificity for prostate cancer
cells is suboptimal: therefore, we are currently developing
related systems with improved selectivity for such biological
targets.
Experimental
Fig. 9 Epifluorescence imaging of prostate cancer (PC-3) cells incubated with 2 µM of probe 1 (15 minutes incubation at 37 °C). Micrographs show: (a) an overlay image of (b) and (c); (b) fluorescence image
with λex = 460–500 nm, long-pass filtered at 510 nm, showing the biolocalisation of 1 throughout the cytoplasm; (c) brightfield image.
The NMR titration results for probe 1 and 2 suggest that there
are two different binding events for fluoride with probe 1. We
propose that both the boron atom and NH fragment can interact with the fluoride anion. From the results with probe 2 the
amine NH proton is simply removed (through hydrogen
bonding/deprotonation), but for probe 1 on addition of fluoride two species initially coexist, which are the free boron and
the fluoride bound boron systems. Adding more fluoride to
probe 1 results in an increase in the amount of fluoride bound
boron. While, the overall intensity of the NH proton slowly
decreases until it disappears when 3.0 equivalents of fluoride
are added, which is the hydrogen bonding/deprotonation of
the amine NH proton with fluoride (Scheme 1).
We then probed the biocompatibility and the potential of
probe 1 for cellular imaging applications, taking advantage of
its strong fluorescence emission signal. PC-3 cells were cultured and treated with 1 following a previously published procedure.37 Our experiments indicate that probe 1 is membrane
permeable and the nuclei appear to remain intact, as shown in
Fig. 9. The fluorescence emission of this compound in vitro
appears to be rather strong, since a final concentration of only
2 µM of probe 1 was needed to be incubated with the cells for
bright images to be obtained. Previously, in our hands, the use
of at least 20 µM of similar naphthalimide probes was required
in typical cellular imaging experiments and under similar
conditions.37
4146 | Org. Biomol. Chem., 2015, 13, 4143–4148
Synthesis and characterisation of probe 1
0.287 g (1.06 mmol) Naphthalimidehydrazine dissolved in
MeOH/DMF and 0.252 g (1.08 mmol) boronate ester aldehyde
was added, stirring at room temperature for 12 hours. Solid
was precipitated out after pouring mixture into ice-water. The
crude product were first tried to purify by recrystallisation then
with flash column chromatography to afford probe 1 (0.04 g)
with a yield of 8%. m.p. 240 °C;1H NMR (300 MHz, (CD3)2SO)
11.67 (s, NH, 1H), 8.98 (s, 1H), 8.49 (d, J = 7.2 Hz, 1H), 8.37
(d, J = 8.5 Hz, 1H), 8.15 (d, J = 7.8 Hz, 1H), 7.78 (m, 3H), 7.57
(t, J = 7.4 Hz, 1H), 7.41 (t, J = 7.3 Hz, 1H), 3.98 (d, J = 7.3 Hz,
2H), 1.63 (m, 2H), 1.36 (s, 12H), 0.93 (t, J = 7.4 Hz, 3H);
13
C NMR (75.5 MHz, (CD3)2SO) 164.0, 163.4, 147.0, 144.9,
140.3, 136.0, 133.8, 131.5, 131.3, 129.5, 129.2, 128.8, 125.6,
125.3, 122.3, 119.1, 111.5, 107.7, 84.3, 25.0, 21.3, 11.8; ESI
Mass [M + H+] C28H30B1N3O4 Calculated 484.2363, found
484.2343.
Synthesis and characterisation of probe 2
0.135 g (0.50 mmol) Naphthalimidehydrazine together with
salicylaldehyde (0.073 g, 0.6 mmol) were refluxed in ethanol
(30 ml) for 5 h. Then the mixture was cooled and recrystallized
from ethanol to afford probe 2 in the yield of 75%
(0.14 g). m. p. 254 °C; 1H NMR(300 MHz, (CD3)2SO) 11.47
(s, 1H), 10.24 (s, 1H), 8.81 (ds, J = 8.6 Hz, 2H), 8.47 (d, J =
6.6 Hz, 1H), 8.37 (d, J = 8.5 Hz, 1H), 7.80 (m, 2H), 7.64 (d, J =
8.5 Hz, 1H), 7.25 (m, 1H), 6.91 (m, 2H), 3.98 (t, J = 7.4 Hz, 2H),
1.62 (m, 2H), 0.90 (t, J = 7.4 Hz, 3H); 13C NMR (75.5 MHz,
(CD3)2SO 164.1, 163.3, 156.6, 146.7, 142.4, 134.0, 131.2, 129.5,
128.6, 126.8, 125.3, 122.3, 120.9, 119.9, 119.0, 116.5, 111.1,
106.8, 41.2, 21.3, 11.8; ESI-Mass: [M + H+] C22H20N3O3+
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Calculated 374.1499, found 374.1518; Elemental Analysis (%):
Calc. C: 70.7, H: 5.13, N: 11.2; found C: 70.1. H: 5.15, N: 11.1.
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Cells culture and epifluorescence microscopy
Prostate cancer (PC-3) cells were grown as monolayers in T75
tissue culture flasks, and cultured in Roswell Park Memorial
Institute medium (RPMI) supplemented with 10% fetal bovine
serum, 1% L-glutamine (200 mM), 0.5% penicillin/streptomycin (10 000 IU mL−1/10 000 mg mL−1). Cells were maintained
at 37 °C in a 5% CO2 humidified atmosphere and grown to
approximately 85% confluence before being split using a 2.5%
trypsin solution. For microscopy, cells were seeded into glass
bottomed Petri dishes and incubated for 24 h to ensure
adhesion. There were two concentration of probe 1 stock solutions prepared, namely: 1 mM and 0.2 mM, in the final
volume they were diluted 100 times into 10 µM and 2 µM
respectively. The optimum concentration used in this experiment was made up of 10 µL of the 0.2 mM probe 1 stock solution (containing 1% of CH3CN) and 990 µL of RPMI serum
free medium. Cells were washed 5 times with 1 mL Hank’s
Balanced Salt Solution (HBSS) and incubated with the 2 µM
probe 1 at 37 °C for 15 min. Cells were washed three times
with HBSS prior to imaging. Epifluorescence imaging was performed on a Nikon Eclipse TE2000-E epifluorescence microscope. It was carried using a mercury lamp (Nikon HG-100W,
Tokyo, Japan) and a high-definition cooled colour digital
camera (DXM 1200C, with 12.6-mega output pixels). Fluorescence images were captured using the GFP-L (green)
channel: λex = 460–500 nm, λem = 510 nm long pass. Images
were collected and analysed via Nikon NIS-Elements software
package.
Acknowledgements
TDJ, SX and XS are grateful for financial support from China
Scholarship Council (CSC) and University of Bath Full Fees
Scholarship. The Natural Science Foundation of China provided support (YBJ). TDJ thanks Xiamen University for a guest
professorship. SIP, RLA and HG thank the EC for an ERC Consolidator grant (O2SENSE) and the Royal Society for funding
as well as the University of Bath for studentships. Dr Colin
Wright (Nikon Bioimaging UK Ltd) and Prof. Stan Botchway
(Research Complex at Harwell) are thanked for helpful discussions and training in the imaging experiment setup. The Catalysis and Sensing for our Environment (CASE) network is
thanked for research exchange opportunities.
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